Vinylidene chloride-vinyl chloride copolymer composition for making shrinkable, translucent films



Aug. 2, 1949 UNITED STATES PATENT OFFICE VINYLIDENE CHLORIDE-VINYLCHLORIDE COPOLYMER COMPOSITION FOR MAKING SHRINKABLE, TRANSLUCENT FILMSCarroll R. Irons, Midland, Mich., asslgnor to The Dow Chemical Company,Midland, Mich, a corporation of Delaware No Drawing. Application June28, 1948, Serial N 0. 35,735

4 Claims.

This invention relates to a specific composition consisting of aparticular vinylidene chlo- It relates particularly for packaging foodsto be frozen,

and are capable of from 30 to 50 per cent shrinkage when heated totemperatures from 85 to 100 C.

Within the past few years the copolymers of vinylidene chloride andvinyl chloride have be:

come industrially important. Some of them are unique by virtue of theirnow well-known crystalline character. This property permits theformation .of strong, flexible films by a process of extrusion,supercooling, and controlled pneumatic stretching, such as is described,for example, by W. T. Stephenson in copending application Serial No.721,738, filed January 13, 1947, now U. S. Patent No. 2,452,080. of thevarious crystalline copolymers of vinylidene chloride and vinylchloride, those containing from 70 to 77 per cent vinylidene chlorideand correspondingly from 30 to 23 per cent vinyl chloride are capable ofthe greatest amount of shrinkage when exposed to such moderatetemperatures as 85 to 100 C. This narrow range of copolymercompositions, accordingly, is useful for the packaging of foodstuffs bythe method described by G. V. Moore and the present applicant incopending application Serial No. 788,973, filed November 29, 1 947. Themethod claimed in said application comprises providing a flexible bagmade of a film of the copolymer of 70 to 77 per cent vinylidenechloride, balance vinyl chloride, enclosing the foodstuff in the bag andheating the assembly to a temperature from 85 to 100 C. to cause the bagto shrink into contact with its contents. It has been found thatcopolymers containing more than 77 per cent vinylidene chloride are toohighly crystalline and hence do not shrink sufficiently at moderatetemperatures for use in the above-described process. Similarly,copolymers containing less than 70 per cent vinylidene chloride, balancevinyl chloride, are almost entirely amorphous and do not have therequired mechanical properties.

The copolymer of from to 77 per cent vinylidene chloride, balance vinylchloride. may be extruded readily when properly modified, but becomesdiscolored during exposure to high temperatures in the extrusion zoneunless a modifying agent is present to serve as a plasticizer and todepress the softening point and increase the thermal stability of thepolymer during fabrication. Since films to be used in contact withfoodstuffs should not be discolored and should not impart any foreignodor or flavor to the foods, considerable care must be exercised in theselection of modifiers for use in connection with the copolymer.

It has been found that very few of the common plasticizers are of anyassistance in extrusion of the identified copolymer, and that when thinextruded articles, such as films, are to be made, there are even feweroperative plasticizers. V ith many such modifiers the heat of extrusionresults in a thermal degradation of the copolymer evidenced by darkeningand by evolution of hydrogen'chloride. Whether or not such decompositionis noted, the resulting products are often brittle, especially at lowtemperatures. Most plasticizing modifiers for the present copolymerexude to the surface of films or other thin articles made from it andgive the article an undesirably oily surface. This is especiallynoticeable in the case of stretched thin films and is particularlyobjectionable in the case of those films intended for use in packagingof foodstufis. When the objectionable oiliness of such exudedplasticizers is considered together with the distinct and commonlydisagreeable odors of the plasticizers, it becomes apparent that thepreparation of a composition suitable for use in the exacting field offood packaging requires special care.

It is among the objects of the present invention to provide a'composition of matter containing the above-identified copolymer and suchmodifiers as may be required to extrude the same easily without thermaldecomposition, which is capable of being extruded and stretched to forman oriented crystalline thin film which is at least translucent so as tomake visible any articles wrapped therein, and is odorless, flexible atlow temperatures, does not exude its modifiers on standing, and iscapable of shrinking from 30 to 50 per cent when ex osed to temperaturesin the range from to C.

Ithas now been found that the foregoing and related objects may beattained by tho preparation of a particular composition consisting of atleast 88.25 per cent, and preferably about 92 per cent by weight, andnot to exceed 95.7 per cent of t e co olymer of from 70 to '77 per centof vinylidene chloride copolymerized with correspondingly from 30 to 23per cent of vinyl isobutyl adipate, 3 per cent of diphenyl,mono-'(ortho-xenyl) phosphate, and 92 per cent by weight of the copolymer of73 per cent vinylidene-f When chloride and 27 per cent vinyl chloride.less than 0.3 per cent of tetrasodium .pryophosphate is empolyed, thecomposition is thermally unstable. When more than 1.75 per cent thereofis used in the present composition extrusion of useful articles isimpossible. When more than the stated maximum of the adipate or of theorganic phosphate plasticizers are used, exudation occurs from thefi1m.- When less than the stated minimum of either plasticizer isemployed, the flow viscosity of the composition is so high thatextrusion is diflicult if not impossible, and the polymer decomposes.The above-defined operative composition is capable of being extrudedeasily, and thin films produced therefrom by the process of extrusionand cold stretching do not exude the contained modifiers, even afterstanding for several months, and have no significant odor. turesprevalent in frozen food lockers and may be used not only to wrap meats,vegetables and other foods for storage at freezing temperatures but alsoto wrap cheese, butter and other foods normally stored at highertemperature, without tainting or otherwise adversely affecting thepackaged goods so far as can be determined by taste, odor, andappearance of the wrapped foods. The films are particularly useful andadvantageous when employed in the shri.nk-packaging process of theabove-identified Moore and Irons application Serial No. 788,973.

The tetrasodium pyrophosphate employed in the compositions is not aplasticizer and does not significantly lower the flow viscosity of theco- Such films remain flexible at temperacent of di-isobutyl adipate, 3per cent of diphenyl mono(ortho-xenyl) -phosphate and 92 per cent of acopolymer made from 72.6 per cent vinylidene chloride and 27.4 per centvinyl chloride. The resulting composition had a softening point of about145 C. as'compared with a melting point of 148 C, forthe unmodifiedcopolymer. The composition was'extruded through a tube-forming orifice2.5-inches in diameter into a water bathat C.-'to=supercool the tubewhich was :then conducted between 2 pairs of pinch rollers,the'secondset operating at a peripheral speed ,3 times. that of thefirst set. The tube was also distended radially between the 2 sets ofrollers by means of a trapped air bubble which increased the tubediameter about 4-fold. The sostretched crystalline film in tubular formwas polymer. When the di-isobutyl adipate and the diphenylmono(ortho-xenyl) phosphate are used as modifiers for the copolymerwithout having cut transversely into various lengths and one end of eachsection of film tube was sealed by flattening the tube and heating theend briefly to 150 C. under moderate pressure. The so-formed pouchesshowed no evidence of thermal decomposition resulting from the hightemperature in the extrusion zone. They had no detectable odor and werefound to contribute no taste or odor to food packed therein by the heatshrinking process. The films made from this composition were flexible attemperatures of 40 C. and lower, they exhibited no tendency to exudeplasticizer on prolonged standing at room temperature, and, whensubjected to a temperature of 85 C. by immersion in hot water, werefound to shrink about 45 to per cent.

I claim:

1. A composition of matter consisting of (A) from 88.25 to 95.7 per centby weight of the copolymer of from to 77 per cent of vinylidene chloridecopolymerized with correspondingly from 30 to 23 per cent of vinylchloride, and (B) correspondingly from 11.75 to 4.3 per cent in theaggregate of the following modifiers, each within thestated range ofproportions: (1) from 0.3 to 1.75 per cent of tetrasodium pyrophosphate,(2) from 2 to 6 per cent of di-isobutyl adipate, and (3) from 2 to 4 percent of diphenyl mono- (ortho-xenyl) phosphate.

2. A composition of matter consisting of 92 per cent of the copolymer of73 per cent vinylidene chloride and 27 per cent vinyl chloride, 1 percent of tetrasodium pyrophosphate, 4 per cent of diisobutyl adipate, and3 per cent of diphenyl mono (ortho-xenyl) phosphate.

3. A translucent odorless'oriented crystalline film which is free fromtendency .to exude its contained modifiers, is flexible at 40 C, and iscomposed of the composition claimed in claim 1.

4. A'translucent odorless oriented crystalline film which is free fromtendency to exude its contained modifiers; is flexible at 40 C. and iscomposed of the composition claimed in claim 2.

' CARROLL R. IRONS.

No references cited.

